1. Field Of The Invention
This invention relates to the prevention of corrosion of ferrous metals in contact with aqueous systems and the reduction of tool wear in metalworking operations.
2. Brief Description Of The Prior Art
Water has long been the coolant of choice in engines, water towers, metal removal operations and other heat generating environments. The obvious problem of corrosion in such systems had for many years been addressed by the use of nitrite salts and nitrite/amine combinations. However, once it became known that the nitrosamines which are formed in such systems pose a health hazard, their use in environments where human contact was involved became severely limited.
One suggested approach has been to employ semipermanent phosphate-metal coatings, the formation of which requires extensive treatment processes, in an effort to reduce corrosion. However, these coatings are ineffective when broken by any metal removal operations.
In metal removal operations, it has been proposed to attempt to both reduce tool wear and provide some measure of corrosion protection by the use of phosphate esters. These phosphate ester additives do not alone provide enough corrosion protection, nor are they as cost effective as nitrite and nitrite/amine systems when combined with other rust preventives such as amines and amine soaps.
Another suggested approach has been to utilize phosphonic acid chelating salts in an attempt to reduce corrosion. The problem which has arisen in the use of phosphonates in the metalworking environment is that they are extremely effective in removing surface deposits and suspending fines, and they therefore remove and suspend rust and corrosion from all metal surfaces which they contact and suspend the fines generated by the metal removal operations. The result is that metalworking fluids which incorporate phosphonates readily transform an initially clear fluid into a brown, nearly opaque suspension. This is not only aesthetically undesirable, but it also causes the reject rate to be quite high for finished parts, since the brown fluid residue which remains on the parts is mistaken for surface rust of the part itself.
Yet another approach has been to employ dibasicdioic acids as rust and corrosion inhibitors in the metalworking environment, but, at the concentration levels ordinarily employed (usually 7 to 10% by weight of a concentrate), they are extremely expensive relative to other materials. Moreover, at such high concentrations, dibasicdioic acids are known to form salts which may contain nitrosamines. Further, experience has shown them to be of relatively low effectiveness and occasionally in rather short supply.
Thus, none of these suggested alternatives appears to offer an entirely satisfactory corrosion prevention system, particularly in the metal removal environment, from a functional, environmental or economic standpoint.